Non-volatile semiconductor memories, such as EEPROM, and flash memory, have become increasingly pervasive in many electronic applications, including flash drives, smart cards, and embedded systems. In order for such non-volatile memories to store data, however, high voltages are needed to write data. Generally, these high voltages are generated by charge pump based power supplies on the same integrated circuit as the memory array. Charge pump power supplies typically use capacitive boosting techniques to create on-chip programming voltages in excess of the externally applied power supply voltage.
In a typical non-volatile memory system, the same charge pump based power supply is used to generate the high voltages required to writing and erasing the memory array, and the lower voltages need to read the memory array. Because shrinking semiconductor geometries have created a corresponding decrease in the amount of voltage a standard semiconductor device can tolerate before breaking down or being destroyed, specially constructed high voltage devices are required whenever high on-chip voltages are used. These high voltage devices are generally larger, slower and less area efficient than lower voltage minimum geometry devices, so area efficient designs minimize the use of such high voltage devices. Consequently, the read circuits for non-volatile memory typically employ low voltage devices that cannot withstand high programming voltages.
In order to prevent damage and breakdown to low voltage devices after a read or erase cycle has been performed, conventional non-volatile memory systems typically discharge the output of the charge pump from its high voltage state to a standby voltage prior to generating a lower voltage. Recharging the output of a charge pump power supply takes time, however, typically in the range of 10 μs to 30 μs. In applications that require high speed, this time delay can slow down system performance. Furthermore, discharging and recharging a charge pump dissipates excess power and energy, which can be disadvantageous to low power applications.
What are needed are fast, power efficient circuits and methods to provide power to systems with multiple voltages larger than the supply voltage.